One well-known thyristor is the silicon controlled rectifier (SCR). The SCR is a unilateral conducting device and has three terminals: an anode, a cathode, and a gate. Applying a control signal to the gate causes the SCR to switch to a low impedance state, which permits current to flow out of the cathode.
In one application where large numbers of serially connected SCRs are used in the control circuit of a telephone switching network, it is desirable to verify the desired operation of the control circuit and to assist in the identification and location of faults therein. One prior art arrangement monitors the total load current of the control circuit to determine how many devices are conducting current, but is unable to identify which devices are conducting current. Another prior art arrangement monitors current flowing to the gates of a group of devices, but is unable to determine how many or identify which devices are conducting current. Still another prior art arrangement detects current flowing out of the cathode of each device to verify the desired operation of the control circuit and to assist in the identifiction and location of faults therein. In this arrangement, an additional wire is typically connected to the cathode of each device to detect current flowing out of the cathode. In applications where large numbers of SCRs are used, these additional wires severely limit the number of individual SCR control circuits that can be integrated into a single integrated circuit package due to the limited number of terminals on the package. This typically increases the cost of the overall circuit. Furthermore, when used with a telephone switching network, these additional wires may also pick up extraneous signals to cause false indications.